This invention relates generally to gas-insulated equipment, and more particularly to a contact for electrically connecting a particle-trapping ring with the outer sheath of a gas-insulated transmission line.
Gas-insulated transmission lines are being used on an ever increasing scale in recent years due to the desirability of increasing safety, problems in acquiring right-of-way for overhead lines, higher power lines required by growing metropolitan areas, and the growing demands for electrical energy. Compressed gas-insulated transmission lines typically comprise a hollow outer sheath, an inner conductor disposed within the outer sheath, a plurality of solid insulating spacers which support the conductor in the sheath, and a compressed gas such as sulfur hexafluoride or the like in the sheath to electrically insulate the conductor from the sheath. Gas barriers may be provided at intervals along the length of the assembly to provide isolation between various segments of the transmission line.
One problem occurring in the use of gas-insulated transmission lines concerns the mobile conducting or semi-conducting particle. These particles, which may enter the line during the fabrication of the line or during installation in the field, cause problems in that they may lower the dielectric strength of the insulating gas and may initiate flash-over and breakdown of the gas as they travel between the outer sheath and the inner conductor. Trump, in U.S. Pat. No. 3,515,939, disclosed a means for deactivating and eliminating the deleterious effects of such conducting particles. In that patent, Trump describes the use of electrodes placed inside the outer sheath to create low field regions which trap and deactivate the particles. Whenever a particle enters the low field region, it cannot acquire sufficient force to propel it out of the region, and thus is trapped in the low field region so that it cannot initite breakdown of the line.
One of the simpliest and most economical means for providing the particle-trapping low field region is to place a ring within the outer sheath which is spaced apart therefrom. This ring typically would have apertures or slots in the bottom regions thereof to facilitate the entrance of conducting particles into the low field region between the ring and the outer sheath. However, to achieve the low field region, the ring must be electrically connected to, and at the same potential as, the outer sheath. This connection of the particle-trapping ring to the outer sheath has, in the past, been through a leaf contact which was physically secured to the ring and which was in sliding contact with the outer sheath. Recently, however, difficulties have been experienced with the use of the leaf contacts. Excessive wear on the contact and the sheath have been observed, and this erosion was sufficient to produce a gap which could insulate the voltage capacitively coupled to the particle trapping ring. Also, the erosion process created a deposit of very fine gray powder which collected along the outer sheath. Additionally, the low force exerted by the leaf spring contact, which is on the order of 0.5 pounds, is insufficient to push the contact through tarnished or oxide layers.